In the discussion that follows, the prior art is set forth in terms of the need for this invention. It is to be understood that we claim invention both in the recognition of that need as well as the solution that follows.
Modern concrete paving practices impose more severe constraints on concrete quality every year. Specifically, concrete when freshly mixed is tested and measured for different desired qualities and standards pursuant to imposed and specified quality control standards. These standards include moisture content (or slump), both compressive and flexural strength after a prescribed number of days, aggregate shape, air content, and uniformity, to name a few. If the quality standards of the concrete produced vary statistically above or below the prescribed standard mean, then the concrete producer is penalized financially.
Exemplary of these standards would be concrete compressive strength where the concrete strength is to reach say 3,500 psi in 28 days. The specification might allow a variation of this standard of 5% above or below this mean or the contractor would be penalized.
It is generally agreed that higher strength concrete can be reached in a shorter period of time by better mixing action and lower water/cement (W/C) ratios. Thus the lower the concrete slump, the easier it is for the contractor to reach the specified strengths. The trend in the industry is toward lower W/C ratios. Low W/C ratio concrete mixed in conventional tilting drum mixers do not reach uniformity as quickly as the mixer used in this invention.
The cost of the concrete makes up the majority of the cost of the road being built. Given the large volumes of concrete processed in such paving contracts, supervisory and specifying authorities such as state and federal inspectors can only statistically sample the loads of concrete to determine the quality of the concrete delivered by the contractor. Because of the large quantity of concrete that can be produced by the contractor in a day, the contractor faces great financial risk if many days pass before he realizes the concrete he is producing is testing outside of specification mean. The above example is intended to show how important it is for the contractor to maintain quality control on the concrete he produces. It is imperative that the contractor use batching and mixing equipment capable of delivering uniformly mixed concrete of the low slump variety to precision construction specifications.
Modern paving practices also call for the use of slipform pavers, which in operation consume relatively large amounts of concrete. On a typical urban size paving job, where the total cubic yards of concrete to be used on the job is relatively small, a modern paver can consume concrete in the range of 240 to 300 cubic yards per hour. On larger jobs the contractor may choose to mobilize, produce and deliver concrete to the slipform paver at a higher rate with a larger plant with higher capacity. Exemplary of such a paver is that Slipform Paver sold under the designation of model S850 built by Guntert and Zimmerman of Ripon, Calif. The fundamental design of this model was pioneered by the late Ronald M. Guntert, Sr. of Stockton, Calif. as set forth in U.S. Pat. Nos. 4,493,584 and 5,135,333.
Other more recent examples of pavers consuming high volumes of concrete can be found in U.S. Pat. No. 5,590,977 entitled Four Track Paving Machine and Process of Transport by Ronald M. Guntert (herein) et al. And U.S. Pat. No. 5,615,972 entitled Paving Machine with Extended Telescoping Members by Ronald M. Guntert (herein).
As cement in the concrete starts to hydrate during transport to a paving site, portable concrete batching and mixing plants have been developed for mixing concrete adjacent the paving site. This reduces the hauling distance to where the concrete is being used and to reduce the number of concrete hauling units required. Simply stated, from a plant which mixes concrete to the site where such mixed concrete is placed, most contract specifications set a time limit of 30 minutes for non-agitating trucks, which is about a 12 mile transport limit. This practical transport limit is reduced in high traffic areas or other situations where the average speed at which the hauling unit can travel is reduced. If the time limit is exceeded, the concrete that is hauled will start to set before the paver places it and the paver placed concrete will not meet the required contract standards.
Secondly, and given the high quality constraints placed on the paved and/or placed concrete product, so-called continuous flow concrete plants have proven inadequate. Such plants are capable of delivering large volumes of concrete but do so on a continuous flow basis. The exacting standards of thorough mixing covered by precise constituent proportion make the continuous flow adjustment of such plants hazardous from the quality control standpoint. As a result, such continuous flow concrete plants have not been accepted in modem paving practice, at least in the North American paving market. It is only the processing of specific "batch" quantities of cement, water and aggregates that constitute concrete that enables the relatively high quality requirements to be maintained and conventional calibration and quality assurance measures to be used.
Prior art portable modem batching and mixing concrete plants are large, require concrete foundations and are difficult to erect, often consuming three to five days in assembly. Frequently, these plants require special rigging equipment, such as cranes to accomplish erection. Specifically, it is not uncommon for such plants to occupy 3 or more (sometimes as many as 7) transporting trailers. Further, such plants utilize rotating and tilting drum mixers located high overhead so they can tilt and gravity feed the mixed concrete into the hauling units. The mixer itself is belt fed with aggregates that are gravity fed through weighing hoppers to maintain precise concrete constituent proportions. This produces several undesirable features, which complicate the erection and subsequent operation of such plants:
First the feeding belt is usually gravity fed from overlying storage bins and weighing hoppers. Thus, considerable weight must be supported at substantial heights from the ground on such portable plants. Using weighing belts instead of weighing hoppers is novel in the U.S. for mixing concrete. In order to load the overlying storage bins that cannot be reached directly by a front end loader, separate charging conveyors with charging bins are used for each aggregate and sand. The charging bins are at an elevation that can be reached by a front-end loader. Because of the requirement of these charging conveyors and bins, the plant site required is quite large limiting the number of places the plant may be set up.
Second, such rotating drums must be tilted, and in a few cases, reversed in rotation for discharge. This tilting superimposes a moment requirement upon the weight support requirement of the rotating drum. As a result of the weight and moment requirements, most so-called portable concrete batching and mixing plants require concrete foundations. Further, in a few cases, reversing the mixing drum rotation not only interrupts mixing, but also consumes momentum, and utilizes heavy reversible drives.
Third, because the rotating mixer drums are supported high in the air, if the more desirable gravity feed of cement is used with the rotating mixer drum, the cement silo must be elevated even higher in the air. The resulting silo and structure requires concrete foundations. To save height, and in lieu of gravity feed from the silo to the cement batcher, many manufacturers of conventional concrete plants use cement screws or air slides to convey the cement into the mixer. Most contractors agree these cement-conveying schemes are undesirable although many times tolerated to minimize the silo height.
Four, because tilting drum mixers are open in front for discharge and open in the back for loading the concrete constituents into the mixer, it is very difficult to suppress the dust that results from the loading operation. The inability to adequately suppress the dust coming out of the mixers limits the use of the plant in many urban settings.
Five, because the tilting/rotating drum mixer rotates on rollers, can be driven by chain drives or gearbox driving gear on drum. The mixer drum is essentially open during the mixing process. As a result, these conventional mixers are very noisy which limits the use of this plant in many urban settings because of the high decibel readings produced.
Six, conventional batching and mixing plants are highly specialized. A contractor will own one plant for his jobs requiring concrete production of 200 to 300 cubic yards per hour and another complete plant when his concrete production needs are 400 to 500 cubic yards per hour. Generally, the larger the plant production capacity per hour the more cumbersome the plant is to transport, set-up and tear down.
Finally, rotating/tilting drum mixers are relatively slow in delivering desired amounts of thoroughly and uniformly mixed low slump concrete, base courses and soil cement. Rotating/tilting drum mixer has paddles affixed to the rotating drum wall. The limitation of this design is that dry material bridges in the mixer and does not discharge out of the drum readily. Moreover, when cement substitutes are used such as slags, the concrete tends to be sticky which again impedes rapid discharge. With low slump concrete or soil cement, this problem is amplified. As compared to contemporary twin shaft, compulsory mixers now utilized in Europe, longer mixing cycles are generally required for the same material in rotating/tilting drum mixers. With low slump or difficult mix designs, rotating/tilting drum mixers produce less than thorough mixing with resultant "ribbons" of less than homogeneously mixed concrete when compared to a compulsory mixer. As a result, considerable additional mixing time or "dwell time" of the concrete in the rotating/tilting drum mixer is required resulting in fewer loads of concrete being produced in an hour.
It should be understood that so-called compulsory mixers are now in use in Europe and in limited use in North America for mixing soil cement. These mixers include a top loading, parallel rotating shafts with interval and paired counter-rotating paddles, and a bottom discharge feature. In the past, such compulsory mixers have been used in the European market where the total transport envelope allowed is small when compared to North America. As a consequence, such compulsory mixers have not been adapted to high volume portable concrete batching and mixing plants used in North America. A plant of the dimensions of this invention would not have been conceived for the European market (or other markets which have adopted European transport standards) because it would not be allowed to be transported on the highways. It should also be noted that the majority of the compulsory mixers used in North America today are foreign made and are used in a continuous mixing process as opposed to a batch mixing process that this invention utilizes.
In understanding the background of this invention, attention should be directed to the practical consequences of having long erection times for portable concrete batching and mixing plants. First, modern slipform pavers can be moved to a new paving site and set-up within one working day (when short transport distances are involved, transport and set-up of the slipform in a day is feasible). Second, current "portable" concrete batching and mixing plants require between three and five days for an equivalent move with 300 to 400 man hours being devoted to each set-up and tear down. The practical result of the time differential between the movement of the slipform paver and the movement of the current so-called portable batching and mixing plant is interesting to understand.
Taking the case of roadway paving of a four lane divided highway, both directions of traffic are diverted to one side of the highway while concrete placement, paving and curing occurs on the opposite side of the highway. Traffic must be maintained while rehabilitating the concrete road. Curing of newly placed concrete on a highway occupies up to 28 days before traffic is allowed on the highway. There is a considerable interval of time where the nearby batching and mixing plant--required to be nearby to reduce the transport interval--will normally remain idle given the total time interval for plant moving. Moving requires 3 to 5 days to set up and 3 to 5 days to dismantle. Thus the decision is frequently made to leave an erected plant idle and in place for paving the opposite side of a highway. Considered from the standpoint of the contractor, the operating hours of a current portable batch plant are about half the operating hours of modern slip form pavers. Stated in other terms, the contractor must either own an additional batching and mixing plant or lose the opportunity to use the slip form paver in performing other work. Given modern capital requirements (including about $850,000 for a "portable" batch plant and $600,000 for a modern slipform paver), neither alternative is desirable.
Finally, there must be considered the dimension of the North American road transport envelope used in Canada, USA, Mexico, and Australia. Maximally, transported loads over high quality highways are normally limited to trailer vehicles having less than 85 feet length overall, 13 feet 6 inches in height, and under 12 feet in width. It will thus be immediately understood that in producing a high capacity batch plant, the size of the transport envelope works against the design. While relative size is not normally a consideration in determining invention, in what follows transport envelope size is a critical design factor in the design of the two trailer transportable, high capacity concrete batching and mixing plant of this invention.